This invention relates to zinc sulfide pigment for use in shaped parts made of synthetic organic polymers, and in coatings such as lacquers, colors, jointing and sealing compounds with good transparency for the range of visible light, good remission of infrared radiation and good fireproofing properties.
From the company brochure no. 1119596 of Sachtleben Chemie GmbH, Duisburg, a zinc sulfide pigment is known, for whose production highly purified zinc salt and sodium sulfide solutions are used. By annealing at about 700xc2x0 C., the very finely precipitated zinc sulfide is brought to the narrow particle size distribution with a central value of about 0.3 xcexcm, which is optimum for the light scattering power. The high remission in the visible and near ultraviolet regions provides a neutral whiteness as well as an optimum efficiency with respect to the scattering and covering power of coatings and an optimum brightening power in plastics. Due to the white pigment properties, the zinc sulfide pigment is used wherever organic or inorganic binders must have a high pigment loading for specific applications, e.g. in undercoat lacquers, jointing and sealing compounds, primers etc. Plastics are likewise pigmented with zinc sulfide, for instance melamine, urea and polyester molding compounds, so that the same can very well be dyed and in addition good fireproofing properties are achieved. The thermal stability of many thermoplastic materials can be influenced favorably by zinc sulfide pigment.
Due to the relatively low Mohs"" hardness of 3 and the spherical particle shape, zinc sulfide has a very low abrasiveness and therefore causes no metal abrasion when it is being processed. Zinc sulfide is used as white pigment in plastics on a large scale, because in contrast to zinc sulfide, the titanium dioxide alternatively used as white pigment leads to the breaking of glass fibers due to its higher Mohs"" hardness of 5.5 to 6.5.
It is the object of the present invention to provide zinc sulfide pigment for use in shaped parts made of synthetic organic polymers and in coatings such as lacquers, colors, jointing and sealing compounds, which not only should have good fireproofing properties, but also a good remission of infrared radiation and at the same time a good transparency in the range of visible light.
This object is solved by zinc sulfide pigment with a mean particle size d50 in the range from 1 to 14 xcexcm with a narrow particle size distribution, which in an amount of 0.5 to 55 vol-%, preferably 1 to 45 vol-%, based on the finished product, is added to the polymers and is produced in that a compound containing sulfide is brought in contact with an aqueous solution containing a zinc compound, and the precipitated zinc sulfide is annealed for 10 minutes to 4 hours at a temperature of 750 to 1200xc2x0 C.
By adding 0 to 30 wt-%, preferably 0.5 to 25 wt-% annealing salt, based on the content of zinc sulfide, the rate of growth of the zinc sulfide pigment particles can be accelerated.
Preferred further aspects of the invention include the following: The zinc sulfide pigment may have a mean particle size d50 in the range from 1 to 12 xcexcm. The process may further comprise the addition of 0 to 30 wt-%, preferably 0.5 to 25 wt-% annealing salt, based on the content of zinc sulfide. The process may comprise the use of one or several of the zinc compounds ZnSO4, ZnCl2, ZnO, Zn(OH)2, or ZnCO3. The process may comprise the use of one or more of the sulfide compounds Na2S, K2S, H2S, or (NH4)2S. The annealing may be conducted in the range from 800 to 1100xc2x0 C. The annealing treatment may be conducted for 30 minutes to 2.5 hours. The process may further comprise quenching in water subsequent to the annealing treatment and a subsequent acid treatment at a pH value from 2.0 to 4.0. The process may further comprise the use of one or more of the annealing salts alkali, alkaline earth and zinc compounds, such as NaCl, KCl, ZnCl2, ZnSO4, and MgCl2. The process may further comprise a comminuting treatment, preferably grinding. The process may further comprise a hydrothermal treatment of a suspension containing the precipitated zinc sulfide at a pressure from 50 to 100 bar and a temperature in the range from 80 to 250xc2x0 C. The process may further comprise a treatment of a suspension containing precipitated zinc sulfide by classifying, preferably by hydrocyclone separation. The process may further comprise a classification of comminuted zinc sulfide, preferably by air-flow classification.
For a better processing in the polymers, the zinc sulfide pigment can be subjected to an inorganic or organic after-treatment, as this is for instance also common practice in the known zinc sulfide pigments as well as titanium dioxide pigments mentioned above.
Synthetic organic polymers include all duromers, elastomers and thermoplastic materials, which possibly can also contain processing aids such as stabilizers, plasticizers, organic and inorganic pigments, dyes, glass fibers etc., singly or plurally.
When using the inventive zinc sulfide pigment in shaped parts made of synthetic organic polymers, there may in addition be used one or more of the fireproofing agents aluminium hydroxide, antimony oxide, magnesium hydroxide, halogenated organic compounds, etc.
The advantages achieved by means of the invention in particular consist in that the shaped parts made of synthetic organic polymers and coatings such as lacquers, colors, jointing and sealing compounds by using the inventive zinc sulfide have a high transparency for the light in the visible region as well as a good remission of infrared radiation. The evaluation of the fireproofing properties is identical with those achieved with the comparatively much more fine-grained zinc sulfide pigments described in the prior art.
In accordance with a preferred aspect of the invention there is provided a content of zinc sulfide pigments of 5 to 35 vol-%, based on the finished product. As a result, particularly good fireproofing properties, transparency in the range of the visible light and remission of infrared radiation can be achieved.
The zinc sulfide pigment has a very low scattering power and no inherent color, so that it leads to a low covering power when it is incorporated in the shaped parts and in the coatings. Therefore, the zinc sulfide pigment can be used in combination with color pigments without occurrence of a noticeable brightening.